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Introduction

1.1 Electrochromism: A Brief Note on the History and Recent Evolution

Electrochromism can be defined as the reversible color change of some specific classes of chemical compounds, both inorganic and organic, occurring upon their electrochemical reduction and oxidation. This phenomenon was reported primarily in 1815 by the prominent Swedish chemist Jons Jacob Berzelius, who observed a color change in heated WO3 when exposed to a hydrogen flux. As further outlined by other influential authors, in 1824, the German chemist Friedrich Wohler, famous for being the first to isolate the chemical elements of beryllium and yttrium in their pure metallic forms and for synthesizing the organic compound urea from inorganic reagents, also known as Wohler synthesis, was fascinated by the beauty of WO3 and its impressive color change upon reaction with sodium which resulted in a gold-like color, likely due to the formation of sodium tungsten bronze [1]. Several other acute observations and notes on the chromism of WO3 and its electrochemical behavior have been continuously reported over the years. In this context, the term electrochromism was coined in 1961 by J. R. Platt, who observed a color change in some conjugated organic dyes upon application of a strong electric field [2]. However, it was only in 1969, with the seminal paper of Satyen K. Deb on the electrochromic (EC) coloration of WO3 thin films, that electrochromism began to follow the first steps toward a more systematic investigation taking into account the technological aspects and the applications [3]. Since then, numerous leading scientists have reported significant works on the basic fundaments of electrochromism, contributing to provide a more comprehensive knowledge of the EC phenomena and the electrochemical processes and to the continuous progress of new materials and devices.

With regard to WO3 and other inorganic EC materials, the renowned book Handbook of Inorganic Electrochromic Material of Claes-Göran Granqvist (1st Edition - March 16, 1995, Elsevier) represents a milestone in the literature of inorganic materials covering electrochromism in metal oxides, material preparation, especially via thin film technologies, characterization methods, electro-optical properties, device design, and performance analysis [1]. Although 30?years have passed since the first publication, it still remains a needful and essential book for a comprehensive and scrupulous knowledge of the fundaments and basic principles of EC inorganic oxides.

Starting with the electrochromism of metal oxides, especially transition metal oxides, such as tungsten, molybdenum, iridium, titanium, manganese, vanadium, nickel, cobalt, niobium, research efforts have also extended to the exploration of the chromism of organic compounds. These include viologens, phenothiazines, and various dyes and pigments such as orthotolidine, anthraquinone, and Prussian blue (PB). Among these, viologen is one of the dyes that have been more extensively studied for electrochromism, despite becoming more popular as paraquat for its herbicidal properties and high toxicity to mammals, including humans (Figure 1.1). Upon exposure, paraquat causes severe inflammation and can potentially lead to severe lung damage with an irreversible pulmonary fibrosis, also known as paraquat lung, with a high rate of mortality ranging between 60% and 90%. Several murders, and especially numerous suicides, have been indeed committed by using this lethal poison, including the case of Isabella Blow (1958-2007), one of the most influential fashion celebrities of all time. Moreover, the use of paraquat has also been associated with the onset of Parkinson's disease in farm workers. Initially produced and marketed with the trade name of Gramoxone in 1962 by Imperial Chemical Industries (Berkshie, England), it was widely used as a herbicide particularly for weed and glass control. However, due to extreme toxicity, paraquat was withdrawn from the market of the European Union in 2007 and is now only used by licensed applicators in the United States. On the other hand, despite its elevated riskiness, it remains one of the most commonly used herbicides worldwide for its effectiveness and its wide availability at low cost. Viologens have also found applications in the EC technology, resulting in one of the most studied classes of EC organic materials. This is due to their ability to operate at extremely low driving voltages, determining high optical contrast, and fast switching dynamics. The electrochemical behavior of 1,1´-dimethyl-4,4´-bipyridinium, methyl viologen (MV), was first reported by Michaelis and Hill in the early 1930s, who observed the violet color of the reduced state [4]. However, it was in the 1980s that viologens were extensively studied for EC applications, exploiting their ability of giving rise to three differently colored oxidation states: dicationic, monocationic radical (violet), and neutral species (red/orange). The fact that the bipyridinium radical of MV is one of the most stable known organic radicals, it allowed the effective preparation of air-stable solids making it suitable for the fabrication of EC devices. However, the low write-erase efficiency, i.e. the percentage of coloration that can be converted back to its original state, of MV in aqueous-based EC devices, along with the high solubility of the dicationic and radical species leads to the poor device durability and operational failure. To address this issue, various strategies have been employed, such as incorporating long alkyl chains on the nitrogen substituents, in order to obtain solid ECs or favoring the interaction between the viologen and an immobilized polymeric surface or electrode. Interestingly, among the various organic materials, viologen systems are particularly promising for industrial applications and, to date, remain the primary organic EC material used commercially.

Figure 1.1 Chemical structure and pictograms of methyl viologen dichloride, more commonly known as the highly toxic and poisonous herbicide paraquat, which is still used in numerous countries for weed and grass control.

Source: USFWS Mountain-Prairie/Flickr/CC BY 2.0.

Several other organic species with EC properties, including pyrazolines, quinones, carbazoles, and phenylene diamines have been proposed in literature as potentially interesting for practical applications. Recently, new small molecules with interesting redox activity and optical properties have been developed, such as thiophene- and furan-based porphyrinoids, triphenylamine derivatives, and tetrathiafulvenes or dibenzofulvene derivatives. In addition to their high optical contrast in the visible range, dibenzofulvene derivatives also exhibit a near-infrared (NIR) electrochromism across a broad spectrum due to optically induced intervalence charge transfer transitions (IVCTs). This makes them particularly promising for further development of devices and smart window technologies.

About PB, it has been widely used by artists and painters from the early eighteenth century to the end of twentieth century for its impressive deep blue color tone and its incredible magnetic and seductive effect. As seen in Figure 1.2, it is the characteristic pigment of Picasso's Blue Period, or the color used by Van Gogh to create a moody, dramatic, midnight blue in many famous works, including "Starry Night," "Starry Night on Rhone," and "Terrace of a Café at Night," PB was also widely adopted by the painters during Baroque and Rococo periods, and it remains today as an important pigment for paints, lacquers, printing inks, and other color uses. An example is the clock faces of the Elizabeth Tower, more commonly known as Big Ben, sits atop the Palace of Westminster (London, England), which was recently restored to its original 1859 color scheme of PB and gold in order to bring the Tower back to the original design and vision by the architects Charles Barry and Augustus Welby Pugin. The first example of electrochromism for PB thin films was reported in 1978 by Vernon D. Neff from Kent State University (Ohio, United States). He deposited a thin film of PB on platinum electrodes using ferric chloride and potassium ferricyanide, demonstrating the reversibility of coloration [5]. In 1982, Japanese chemists K. Itaya and K. Shibayama developed a more reproducible method for the deposition of PB thin films through the electrochemical reduction of ferric-ferricyanide solution [6].

Figure 1.2 Prussian blue pigment used (a) in Van Gogh's Starry Night on the Rhone (1888, Musée d'Orsay, Parigi) and (b) in The Old Guitarist of Pablo Picasso (1903, Art Institute of Chicago).

Source: Musée d'Orsay/Wikimedia Commons/Public Domain, Flickr/Public Domain.

After these pioneering works, extensive research has been carried out on the structure, chemico-physical and electrochemical properties, and electrochromism of PB and its analogs. Significant results have also been reported on energy-storage properties for applications in batteries and supercapacitors [7-9]. PB undergoes three color changes passing from colorless to blue and then to brown, depending on its redox state. The fully oxidized brown state is unstable, and for practical EC applications, only the reversible switching between colorless and blue can be used, which is associated with the insertion or extraction of balancing cations (preferentially K+ or NH4+). Due to its three-dimensional (3D)...